The present invention relates to apparatus for reducing noise in an image signal.
Conventional apparatus for reducing noise in image signals employ a variety of filtering techniques, such as a three-dimensional processing technique which may be carried out using an apparatus such as that illustrated in FIG. 1. In the apparatus of FIG. 1, an input signal, such as a digital image signal, is received at an input terminal 111. The input signal is supplied both to a still picture noise reducing filter 112 and a moving picture noise-reducing filter 113 each of which reduces the noise content of the input image signal and supplies a filtered, noise-reduced output image signal to a respective fixed terminal of a switching circuit 120.
The input image signal received at the terminal 111 is also provided to a motion detection circuit 130 which serves to detect motion in the input image signal on a pixel-by-pixel basis. The motion detection circuit 130 produces a motion detection signal which it supplies as a control signal to the switch 120 in order to control the selection of the output of the filter 112 or the filter 113 to be supplied at an output terminal 121 of the FIG. 1 apparatus. That is, when the motion detection circuit 130 determines that the input image signal is a still picture, the output of the still picture filtering circuit 120 is supplied to the output terminal 121, while when the motion detection circuit 130 determines that the input image signal is a moving picture, the output of the moving picture filter circuit 113 is supplied at the output 121.
Accordingly, the apparatus of FIG. 1 adaptively responds to motion in the input image signal for reducing its noise content.
The still picture noise-reducing filter 112 is a mean value filter which serves to smooth the input image signal along the time axis. The filter 112 includes a frame memory 115, multiplication circuits 116 and 117, and an adding circuit 118. The frame memory 115 outputs the data of a frame preceding that of the input image signal to the multiplication circuit 116 wherein the output data is multiplied by a factor k, where 0&lt;k&lt;1. The input image signal is multiplied in the multiplication circuit 117 by a factor (1-k). The outputs of the multiplication circuits 116 and 117 are supplied to the adding circuit 118 which serves to add these signals and supply the same to the output of the filter 112 as well as to the frame memory 115 for storage therein.
The moving picture noise-reducing filter 113 is a median-value filter or median filter which is a kind of spatial filter. The filter 113 produces each pixel at a value equal to the average of the value of the input pixel and those of the immediately preceding and succeeding pixels.
A problem encountered in the use of the FIG. 1 apparatus is that the picture deteriorates as a result of filtering, especially at the edges between objects or other image features. A further conventional technique has been proposed wherein it is attempted to locate such edges and then preserve edge information by modifying filter characteristics at edge locations. However, the ability to detect such edges is limited. For example, noise can be mistaken for an edge, and as a consequence, such noise is not removed.
The characteristics of the noise present in an image signal depend on the source of the noise, so that the noise characteristics vary depending upon the particular image signal processing system employed. It will be seen, therefore, that conventional noise-reducing techniques are limited by their inability to adapt fully to the shape or configuration of the picture represented by the image signal, or to the particular image signal processing system.